The invention relates to a vacuum coating unit and furthermore to a method for the simultaneously coating of several planiform or laminar substrates with a hard material coating.
There are PVD vacuum coating units with substrate mountings for tools such as machine tools, cutting tools like drills, cutting inserts, etc., which are optimized primarily for rotationally symmetric workpiece geometries, such as, for example, for shaft tools with different dimensions. Examples of this are production systems by OC Oerlikon Balzers AG Liechtenstein, such as the unit of type RCS described in EP 1 186 681 A1 and the unit of type BAI 1200 described in detail in EP 0 886 880 B1. Typical rotating mountings for indexable cutter inserts (cutting tools) utilized in these production systems are depicted in FIGS. 1a and 1b. The indexable cutter inserts 7 can herein be fastened, for example, on drum-like magnetic workpiece carriers 40 or be disposed on rods for tool receiver 27 and be disposed alternating with spacer pieces 39.
For the coating of small parts are known PVD units in which small parts rotate as bulk goods in grid drums and are thereby moved, while they are simultaneously exposed to the coating from cathodes disposed as the coating sources outside or inside the drum. Such methods, such as described for example in EP 0 632 846, have the disadvantage that through the movement of the drum the small parts impact one another or the drum and, consequently, especially in the case of hard metal parts, surfaces are scratched and sharp edges, such as cutting edges, are damaged.
CVD—coating units for cutting tools, such as indexable cutter inserts, have long been known. A typical example of such a unit type, in which indexable cutter inserts are laid out in grids and are coated in one or several planes is disclosed in WO 99/27155 A1, FIG. 4a of the reference. The chemical process for the deposition of the desired material out of the gas phase can either be excited only thermally or, as in the present document, additionally through a plasma, such as a pulsed plasma, applied between substrates and electrodes.
The applications CH 00518/05 and CH 1289/05 disclose pulsing the arc current either by simultaneously applying a DC and a pulsed power supply to an arc vaporizer source, or by applying a single pulsed power supply to two DC-fed arc vaporizer sources. In this way, several arc sources can be continuously and stably operated even if they are operated in high oxygen-containing or pure oxygen atmosphere and their surfaces are coated with an insulating layer during the process. It becomes thus possible to produce insulating, in particular also oxide, layers in PVD batch production plants.
Currently industrial PVD units for the coating of tools and structural parts are customarily not laid out such that they are only optimized for one substrate form and size. The reason for this is that in these coating systems, for reasons of economics and productivity, a multiplicity of very different substrate sizes and shapes must be coated and that for the PVD layers, customary up to now only thickness ranges from approximately 4 μm up to approximately 6 μm are targeted or that these can also not be produced at greater thicknesses due to the high residual stresses occurring in this case. In order to provide the workpieces with often complex, three dimensional structures uniformly with a layer system of several micrometer (μm) thickness, a multiple substrate rotation is therefore conventionally a requirement. However, this, in turn, leads to the fact that therefore in such methods only relatively low growth rates of a few μm/hr can be attained and therefore PVD units currently have relatively large coating chambers in order to make economic operation possible.
One disadvantage of such units, which in terms of substrate size and shape are universal, is the loading and unloading of the substrates into and from the mountings and into the unit. The demand for universality with respect to the substrates involves more likely an adaptation of the substrate mountings to the unit rather than to the substrates and thereby makes automation of the loading and unloading of the substrates difficult.
There are further significant disadvantages resulting from the universality demand. The dense packing of the substrates in the PVD production system and the rotation necessitated thereby continuously interrupt periodically the directed material flow of the PVD sources toward the substrate, while the supplied reactive gases act continuously onto the layer. There are approaches of disposing additional PVD sources centrally in PVD coating systems in order to relieve the problem. While this reduces the problematics somewhat, however it does not really resolve them since here also the material flow cannot be maintained at adequate constancy over time, at least not under the demand of high loading density at high productivity. The variation in the material flow of the PVD sources toward the substrate leads to a submultilayer structure in the layer build-up, thus to a variation of the structure or composition of the layer over the layer thickness. This can be advantageous, for example in view of the stress inclusion into the layer, however, it also entails disadvantages if very thick layers must be produced. This submultilayer structure depends primarily on the geometry of the substrate mountings. At the state of the present prior art the disadvantages outweigh the advantages and the coating with PVD batch plants is not economic due to the coating rates which are too low, in particular in view of thick oxide layers.
As a further highly important disadvantage of current PVD coating technology should be considered the layer thickness distribution on the tool. This will be explained in detail using indexable cutter inserts (depicted schematically in FIG. 2), however, but applies analogously also to all cutting tools which have cutting faces in different planes and are substantially of two-dimensional geometry. In the case of a mounting of the indexable cutter inserts for double or triple rotation, it is nearly impossible to generate at justifiable expenditures, for example, a uniform layer thickness on flank and rake faces, much less realize a given layer thickness ratio. For this realization to be successful, the freedoms under rotating operation in a batch plant are too severely restricted and such requirements can be neither fulfilled at defensible expenditures through an economic substrate rotation nor through a movement of the PVD sources.
This is one of the reasons for the coating of indexable cutter inserts with layers greater than approximately 6 μm that, for reasons of economy, primarily CVD methods have become widely established, which are capable of cost-effectively coating large batches (charges) with indexable cutter inserts in large-volume CVD coating systems in spite of moderate CVD coating rates. The CVD approach was until recently additionally supported by the fact that there was no PVD production technique available for the oxide production for indexable cutter inserts and only CVD appeared to be possible for this purpose. An important characteristic of CVD coating is the extensively uniform distribution of the layers over the indexable cutter inserts or the regions of the cutting edge, which in many cases is of advantage. However, it should here also be noted that this advantage becomes a disadvantage if a specified layer thickness ratio of flank and rake face must be realized on an indexable cutter insert. And, lastly, the high process temperatures in the CVD approach are not suitable for all tools and are therefore undesirable.
However, the manner in which the indexable cutter inserts are loaded and unloaded for the operation in the CVD coating system is significantly more efficient than in the PVD systems. This rests substantially on the fact that the indexable cutter inserts are laid out on plate-shaped grids. This approach to the substrate handling is primarily also determined by the preceding and succeeding fabrication steps, such as the sintering, the face, side and edge grinding, the sandblasting, polishing, etc., which are reasonable in small lot sizes of approximately 20 to 400 and whose machine working infrastructure is laid out for these lot sizes. The substrate handling accordingly is aligned in the CVD technique with the above stated lot sizes and only in the coating are 5 to 30 of such lots typically combined in one CVD batch for reasons of productivity.
Apart from the low coating rates, the diminished flexibility in the material selection among the coating materials, whose supply takes place via gaseous precursors, has been found to be a disadvantage in the CVD technology. For one, the availability of the appropriate precursors is limited, for another, rare precursors entail high production costs. Added to this is the fact that the gaseous precursors for certain materials can only be handled with difficulty so that the chemical reactions cannot be controlled as freely and independently of one another as is the case with PVD sources, and that CVD reactions must be regulated via the temperature and a greater multiplicity of the precursors in the process chamber impedes the control of the desired reaction. These are all reasons for the fact that using this technique only TiC, TiN, TiCN and Al2O3 layers could be produced until now. TiAlN layers, such as are, for example, possible in PVD without encountering problems and which have large advantages in many cutting applications, have so far not found their way into the standard CVD technology.
In conclusion, the disadvantages of the existing coating techniques can be summarized as follows:
PVD:
                1. Unsuitable substrate handling for large batches of identical, primarily small two-dimensional substrates such as, for example, indexable cutter inserts.        2. Coating rates that, due to the necessary substrate rotation in large batch plants, are too low.        3. Rotation-dependent interruption of the material flow of the solid source toward the substrate.        4. Nearly impossible setting of the layer thickness ratio between flank and rake face.CVD:        1. For reasons of economy, necessity of batch plants due to lower coating rates and long heating and cooling cycles.        2. Flexibility of the CVD process approach with respect to different materials since the precursor selection is restricted and the reaction mechanisms can essentially only be controlled via the process temperature.        3. Complicated process development for new materials and material combinations with high costs when using new precursors.Conclusion:        
It was recently possible to produce metal oxides by means of production-worthy PVD technique. However, in batch plants only low coating rates can be realized due to the necessary rotation, which is suitable for universal substrate sizes, however not specifically for indexable cutter inserts. The existing prior art is based on a rather unsuitable system with unsuitable substrate mounting or substrate handling, whereby the PVD technique is inferior in productivity to the CVD technique in certain application fields, which require especially thick layers and which make a very simple, partially automated handling of indexable cutter inserts reasonable. Furthermore, in PVD batch plants, for reasons of economy (as high a loading density as possible), the indexable cutter inserts most often must be mounted such that the flank face compared to the rake face is coated with thicker layers. This approach has in the past tended to support the specific usage feasibility of the indexable cutter inserts only for milling purposes, however, it is not a preferred approach for turning applications.
The CVD coating rates are low and the heating and cooling cycles long, giving rise to the necessity of large batch plants. The high temperatures and the inflexibility in the materials are disadvantageous. The combination of many lot sizes into one batch increases the process risk, interrupts the substrate fabrication flow and reduces the process control. The CVD technique is thereby limited and at least entails high costs for the development of new materials provided this is even possible at all.